In a typical computer system, numerous signal lines are used to connect integrated circuits. Information is transmitted from a driver to a receiver over these signal lines through different signal levels, with the ability to switch from one signal level to another often being the factor limiting the maximum rate of information transmission. The ability to switch a signal line is not only affected by the characteristics of the driver that is causing the transition, but also by the characteristics of the signal line, the other devices attached to the signal line, and the termination, if any, of the signal line. One important characteristic of drivers, signal lines, and terminations is their impedance.
At least one integrated circuit coupled to a signal line typically has an output buffer for driving the signal line. As technology progresses, these output buffers are able to drive signals having faster transitions. The higher frequency components inherent in the faster transitions are well known to increase various transmission line effects including ringing and wave reflection. These transmission line effects increase as signal frequency increases due to the frequency dependence of signal line impedance. Thus, increasing measures are required to compensate for transmission line effects as signal frequencies are driven higher.
To adequately accommodate high frequency signals, many prior art signaling schemes drive a signal line terminated in a very high impedance input buffer with an output buffer having a driving impedance substantially matched to the characteristic impedance of the signal line. The matching of the driving impedance of the output buffer to the signal line allows the most power transfer from the output buffer to the signal line. Due to the very high impedance of the input buffer, the signal line may be considered to have an "open" termination.
In this signaling scheme, a high signal level (Vcc) and a low signal level (Vss) are used, the low signal level typically being ground (zero volts). A low to high transition generated by an output buffer of a driver device propagates down the signal line as an incident voltage wave until it reaches the end of the signal line after a propagation delay, T. Since the output buffer and the signal line are designed to have approximately the same impedance, the voltage which initially propagates down the signal line is one half the Vcc voltage level. Due to the mismatch between the line impedance and the high impedance at the line end, a reflected wave occurs which propagates back to the output buffer after a total propagation delay of 2T. It is this reflected wave which brings the line to Vcc. Thus, a receiver at the end of the signal line receives a voltage of Vcc after a delay of T, but a receiver near the output buffer does not receive the full Vcc voltage until a delay of approximately 2T. This type of signaling scheme is known as reflected wave switching and has the disadvantage that it requires a time period of twice the propagation delay of the signal line to insure that all receivers have received the appropriate voltage level.
By using incident wave switching, the time required for the entire signal line to attain its final value can be reduced by a factor of two. Incident wave switching can be accomplished in a system with "open" ended signal lines by using a very low impedance output buffer. In such a system, the impedance of the output buffer of the driver device is selected to be much lower than the characteristic impedance of the signal line. When the output buffer switches, an incident voltage wave of practically the full magnitude of the voltage swing between Vcc and Vss is driven on the signal line, allowing receivers to switch upon receiving the incident wave. The reflections which will result from the "open" termination can be limited by clamping circuits included on devices coupled to the signal line; however, these reflections may still result in malfunction due to the overdriving of some devices. Also, an initial reflection from the high impedance end of the signal line may be again reflected by the driver, resulting in multiple reflections with the possibility of causing receivers to sample improper signal levels. Finally, as switching frequencies and edge rates increase, reflections and other transmission line effects in a system with "open" ended signal lines tend to worsen.
Due to the problems posed by reflections on signal lines with "open" terminations, a variety of signal line terminations have been used. One common signaling scheme terminates a signal line to a fixed potential with a termination resistor having the same impedance as the characteristic impedance of the signal line. The signal lines may be terminated to Vcc, ground, or an intermediate potential through the terminating resistor. Using these matched terminations prevents the incident wave from being reflected, thus allowing the receivers to properly sample the signal line upon receiving the incident wave (i.e. incident wave switching). Assuming that the termination resistor is connected to Vcc, a high logic level will appear on the signal line as Vcc, and a low logic level will appear on the signal line as 1/2 Vcc. While the voltage swing is reduced as compared to the "open" termination case, the dramatic reduction in reflections and the ability to utilize incident wave switching make resistively terminated signal lines a practical solution for some applications.
The resistively terminated signal lines discussed above have the disadvantage of increasing power consumption. With the resistive termination to Vcc, driving a signal line to ground will result in a continuous current from the output buffer through the resistor in order to maintain the signal level at 1/2 Vcc. Considering the increasing importance of power savings in computer systems the increased power consumption of a typical resistive termination scheme is a significant disadvantage.
FIG. 1 illustrates a prior art signaling scheme which may reduce some of the problems with resistively terminated signal lines. This scheme is also addressed in "A Dynamic Line-Termination Circuit for Multireceiver Nets", IEEE Journal on Solid-State Circuits, Vol. 28, No. 12, December 1993. Here the driver and receiver combination immediately terminates a line to signal values as they are received. Driving device 100 includes an output buffer 120 which drives a transition on signal line 130. The output buffer 120 is configured with a lower impedance than the signal line 130 such that an incident voltage wave produced by output buffer 120 on signal line 130 crosses the threshold voltage of input buffer 150 of receiver 110. This scheme utilizes an active termination device formed by P transistor 140 and N transistor 160 which is matched to the signal line impedance. The active termination device turns on during the switching of the signal line 130 when the incident voltage wave causes input buffer 150 to switch. Since the termination device terminates the signal line to the signal value being driven, there is not continuous additional power consumption due to the termination; however, the switching of P transistor 140 or N transistor 160 during the incident voltage wave from output buffer 120 can produce a step voltage on signal line 130.
In a typical application, many receivers such as receiver 110 are used on signal line 130, and each termination will contribute to signal line noise with its step voltage, resulting in the incident voltage wave being altered by each of the terminating receivers. These noise causing step voltages occur at different times since the receivers are generally located at different distances from the output buffer. Additionally, once all of the receivers have switched and are terminating the signal line to its present value, the next time a driver tries to drive the opposite value on the signal line, it will have to overcome all of the terminating receivers. This may result in poor performance in a system with many receivers. Further, since the driving circuit is not tuned to the impedance of the line, any noise propagating back to the driver will be reflected rather than absorbed. Thus the signaling scheme shown in FIG. 1 addresses the power consumption disadvantage of resistively terminated signal lines, but has some shortcomings of its own.